If the adjective follows, then you sleep on a big bed, as opposed to a small bed. If it precedes, then you sleep on a bed (which just so happens to be big) as opposed to sleeping elsewhere, perhaps the floor.

I'm getting confused about when to put the adjective before the noun and when to put it after. A few days ago, "I want a big elephant" was "Quiero un gran elefante", but when I used "una gran cama" for "a big bed" in this sentence, Duo marked me wrong and told me it had to be "una cama grande". So is it about masculine/feminine? animate/inanimate? something else I'm missing?

I think Duo are right here but probably wrong about the elephant. Un gran elefante is a great elephant, un elefante grande is a big elephant. Of course, great can mean big too. A big bed is una cama grande. La gran cama is indeed the great bed - I wonder if a Spanish child might refer to their parents' bed in that way ?

Ok so I got everything right except for the word 'Una' and how the spanish version of the sentence was written. Why is it 'Yo duermo en una cama grande' instead of 'Yo duermo en una grande cama' ?????????????????????????????